Photovoltaic (PV) solar energy collection devices used to generate electric power generally include flat-panel collectors and concentrating solar collectors. Flat collectors generally include large-area PV cell arrays and associated electronics formed on semiconductor (e.g., monocrystalline silicon or polycrystalline silicon) substrates, and the electrical energy output from flat collectors is a direct function of the area of the array, thereby requiring large, expensive semiconductor substrates. Concentrating solar collectors reduce the need for large semiconductor substrates by concentrating light beams (i.e., sun rays) using solar concentrator optics (e.g., parabolic reflectors or lenses) that focus the incident light beams, creating a more intense, focused beam of solar energy that is directed onto a relatively small PV cell. Thus, concentrating solar collectors have an advantage over flat-panel collectors in that the PV cells utilize substantially smaller amounts of semiconductor, thereby reducing overall manufacturing costs. Another advantage that concentrating solar collectors have over flat-panel collectors is that they are more efficient at generating electrical energy because they can economically employ higher efficiency PV cells that have a higher cost per unit area due to a more complex structure and different materials.
FIGS. 12 and 13 are exploded perspective view and cross-sectional side views showing a simplified conventional Cassegrain-type concentrating solar collector 50 that includes a PV cell 51 and a solar concentrator optical system 52 including a primary mirror 53 and a secondary mirror 54 that reflect and focus light beams LB through a central opening 53A of primary mirror 53 to form a focused beam 55 (i.e., a region of peak irradiance or power per unit area). As indicated in FIG. 13, primary mirror 53 and secondary mirror 54 are supported on a frame (not shown) such that incident light beams LB are focused to form a high intensity focused beam 55 at a predetermined image plane IP. PV cell 51 is mounted on a structural support or stage 56 that maintains PV cell 51 in the image plane of solar concentrator optical system 52 such that PV cell 51 coincides with focused beam 55.
FIG. 14 is an irradiance (light power per unit area) plot showing the characteristic circular light distribution associated with the image of the sun formed by a focused beam 55 generated by conventional Cassegrain-type concentrating solar collector 50. The angular diameter of the solar disk is approximately 0.54 degrees. Note that the light distribution of focused beam 55 is at the maximum (uppermost) end of the range chart in the central (cross-hatched) region of the irradiance plot, and drops to the minimum (lowermost) end of the range chart abruptly. Note also that, when generated by standardized integrated circuit fabrication techniques, PV cell 51 is square or rectangular, and as such does not match the shape of conventional focused beam 55.
Because PV cell 51 accounts or a significant portion of the overall cost of concentrating solar collector 50, there is a significant incentive to minimize the size (and, hence, the production cost) of PV cell 51. However, the power generated by a particular PV cell is generally related to the total amount of the incident light on the PV cell. As such, in order to reduce the size of PV cell 51 while maintaining the same power output, solar concentrator optical system 52 must be modified in a way that decreases the size of focused beam 55, which also increases its intensity.
For example, as depicted in FIG. 16(A), assume the solar concentrator optics of a conventional concentrating solar collector generate a relatively large, low intensity focused beam 55A that is optimized for a relatively large PV cell 51A. Usually the cell size is designed to be much larger than the size of the focused beam to capture all of the light and accommodate the effects of fabrication tolerances and pointing errors. Because essentially all of focused beam 55A is directed onto PV cell 51A, PV cell 51A generates electrical power that is proportional to the total solar energy transmitted in focused beam 55A. In contrast, if relatively large PV cell 51A is replaced with a relatively small PV cell 51B, PV cell 51B generates electrical power that is proportional to only a portion of the solar energy transmitted in focused beam 55B (i.e., as indicated in FIG. 16(A), a large portion of focused beam 55A misses PV cell 51B and strikes support 56). In order for relatively small PV cell 51B to generate power that is comparable to the PV cell 51A/focused beam 55A combination, the solar concentrator optics would need to be modified to such that the incident light beams are focused onto a smaller area, thereby forming a relatively high intensity focused beam 55B that is substantially the same size or smaller than PV cell 51B.
A problem associated with concentrating solar collectors is that a trade-off is required between the size of the PV cell and the acceptance angle of the solar concentrator optics. The acceptance angle of a concentrating solar collector is the angle of the incident light beams, relative to the optical axis, at which power generation by the PV cell falls below its maximum value.
Referring to FIG. 15(A), when the incident light beams LB are substantially parallel to the central optical axis X of the solar concentrator optics (see FIG. 15(A)), both low intensity focused beam 55A and high intensity focused beam 55B are substantially centered on relatively PV cell 51A (as shown in FIG. 16(A)). Thus, incident light beams LB that are substantially parallel to the central optical axis X are within the acceptance angles of both low and high resolution solar concentrator optics because PV cell 51A collects all of the light from both low intensity focused beam 55A and high intensity focused beam 55B.
As the position of the sun changes from an optimal position illustrated in FIG. 15(A) to a non-optimal position, the direction of the incident light assumes an incidence angle θ1 relative to the optical axis X (as shown in FIG. 15(B)). As indicated in FIG. 16(B), due to the optics required to form high intensity focused beam 55B, incidence angle θ1 causes focused beam 51B to move at least partially off of PV cell 51B, thereby causing the power generated by PV cell 51B to drop below the maximum value. Thus, incidence angle θ1 is greater than the acceptance angle of the solar concentrator optics needed to generate high intensity focused beam 55B. Similarly, as also shown in FIG. 16(B), when the solar concentrating optics are arranged to generate low intensity focused beam 55A, the amount of movement of focused beam 55A in response to incidence angle θ1 is more pronounced, and because the irradiance area of the image is larger, PV cell 51A collects less of focused beam 55A, and therefore does not maintain the maximum power output. Thus, incidence angle θ1 is not within acceptance angle of the solar concentrator optics needed to generate low intensity focused beam 55B.
The basic example illustrates that solar concentrator optics must maintain a small spot size and have a reasonable acceptance angle to insure that all of the light underfills or critically fills the PV cell for maximum power output. The acceptance angle is typically much less than the angle traversed by the sun. This is addressed by incorporating a positioning system that adjusts (e.g., tilts) the concentrating solar collector to “follow” the sun throughout the course of a day to keep the sun within the acceptance angle. If the acceptance angle is very small, the increased tracking accuracy needed greatly increases the overall cost of producing and maintaining a concentrating solar collector array. Conversely, when solar concentrator optics are designed to increase the acceptance angle without having to increase the PV cell size, the associated costs can be lowered. If not, either the power generation is reduced, or a larger (and more expensive) PV cell is needed.
Another problem associated with the use of solar concentrator optics that are adjusted to increase the concentration of light (i.e., to produce high intensity focused beam 55B) is the high light concentration also results in high peak intensities of stray off-axis light as the light distribution no longer falls on PV cell 51B, and begins to fall on support 56. As indicted in FIGS. 15(C) and 16(C), when the light beams LB are directed at a relatively large incidence angle θ2 relative to the optical axis X, focused light beam 55B is located entirely off of (next to) PV cell 55, whereby the high intensity solar energy is transferred to support 56. To address such stray light issues that may arise as a result of high intensity focused beam 55B striking support 56, support 56 must be provided with baffling, thermal management structures, or extensive heat sinking to avoid potentially catastrophic failure modes.
Referring again to FIGS. 12 and 13, another problem with conventional concentrating solar collectors, such as solar collector 50, is that they are expensive to produce, operate and maintain. The solar collector optics (e.g., primary mirror 53 and secondary mirror 54) used in conventional collectors to focus the light beams are produced separately, and must be painstakingly assembled to provide the proper alignment between focused beam 55 and PV cell 51 (i.e., such that focused beam 55 is centered on PV cell 51). Further, over time, the reflectors and/or lenses can become misaligned due to thermal cycling or vibration, causing focused beam to become misaligned (e.g., as depicted in FIGS. 16(B) and 16(C)), and become dirty due to exposure to the environment, thus reducing the intensity of focused beam 55. Maintenance in the form of cleaning and adjusting the reflectors/lenses can be significant, particularly when the reflectors/lenses are produced with uneven shapes that are difficult to clean.
What is needed is a concentrator-type solar collector that maintains or increases the acceptance angle without having to increase the size of the PV cell. What is also needed is a concentrator-type solar collector that provides the increased acceptance angle while avoiding the expensive assembly and maintenance costs associated with conventional concentrator-type solar collectors.